Full Idea
What needs to be stressed is that in both of Hume's definitions of cause, an individual sequence of events is deemed causal only because something extrinsic to the sequence occurs (be it conjunctions, or a mental link).
Gist of Idea
In both of Hume's definitions, causation is extrinsic to the sequence of events
Source
comment on David Hume (Enquiry Conc Human Understanding [1748], VII.II.60) by Stathis Psillos - Causation and Explanation §1.9
Book Reference
Psillos,Stathis: 'Causation and Explanation' [Acumen 2002], p.50
A Reaction
Simple but important. Hume's basic claim is that there is no 'causation' in events, apart from the events themselves. Hence no necessity, on top of the apparent contingency.